1. Field of the Invention
The present invention relates to a toner for developing electrostatic charged images and a developer for developing electrostatic charged images used for forming electronic photographs in electrophotographic and electrostatic recording processes, and the like, and an image forming method using the same.
2. Description of the Related Art
An electrophotographic process is a process comprising: developing an electrostatic latent image formed on the surface of a latent image bearing body (photoreceptor) with a toner containing a colorant, transferring the toner image onto a surface of a recording medium; fixing the image thereon with a fixing means such as a heat roller or the like; and additionally removing the toner remaining on the latent image bearing body after transfer and cleaning the bearing body before forming the next electrostatic latent image once again. Dry-type developers used in these electrophotographic processes and the like are grossly classified into one-component developers, wherein a toner contains a mixture of binder resin and a colorant and others is used alone, and two-component developers containing a mix of both a toner and a carrier. The one-component developers can be further classified into: one-component magnetic developers, containing a magnetic powder that is carried by the magnetic force of a developer bearing body for development; and one-component nonmagnetic developers, not containing a magnetic powder, but which are that is electrostatically charged by a charging means, such as a charging roll or the like, and thus carried by a developer bearing body for development.
From the latter half of the 1980s, in the trend toward digitalization, there have existed strong demands in the electrophotographic market for miniaturization and higher-performance. With regard to the quality of full color images, higher image quality has been desired, up to a level similar to high-quality printing and photographs. In addition, in regard to the quality of black and white images, there exists demands for higher image quality and at the same time, for higher-productivity, miniaturization, and cost reduction. Digitalization of processing is an indispensable tool for obtaining high quality images. A stated advantage of digitalization in regard to high quality image is, for example, that complicated image data can be processed at higher speed. By digitalization, characters and photographic images can be processed separately, and the reproducibility of both has been improved significantly compared with analog technology. Particularly in regard to photographic images, the enabling of gradation and color corrections, and digital processing is more advantageous in contrast, definition, sharpness, color reproducibility, and graininess of images than analog processing. An electrostatic latent image formed in the optical system reads to output faithfully as a printed image, and accordingly the particle size of toners used continues to get smaller and smaller in size in order to faithfully reproduce the electrostatic latent images. On the other hand there are demands for; reduction in the number of parts, for the purpose of miniaturization; and for the elongation of the life of consumables, for the purpose of cost reduction, this makes it is necessary to improve the performance and reliability of the developers. Further, the speed of the latent image bearing body is increasing in order to improve productivity, and thus for obtaining higher-quality images consistently, it is becoming extremely important to improve each of the process of development, transfer, fixing, and cleaning. At the same time, it is becoming important to improve the performance, such as the life of consumables by means of components of the toner.
In particular for obtaining higher-quality images, it is necessary in the transfer process to transfer a developed toner image more faithfully, but toners having a smaller diameter often decrease transfer property. Accordingly, various techniques have been reported for utilizing such smaller-diameter toners more efficiently. For example, a method of improving the transferring performance of a toner by making the toner more spherical is disclosed (see e.g., Japanese Patent Application Laid-Open (JP-A) No. 62-184469). In such a case, making the toner more spherical may improve transfer efficiency, but it leads to improper cleaning due to a small amount of the toner remaining thereon after transfer. Alternatively, a cleaner-less system wherein the toner remaining on the photoreceptor surface after transfer is recovered at the same time as the development of images in the developing device is proposed (see e.g., JP-A Nos. 2-302772 and 5-94113). However, due to the difference in electrostatic property between recovered and fresh toners, the recovery of the residual toner at the same time as the development generally causes problems, such as accumulation in the developing device of the recovered toner, which is less easily developed. This consequently leads to deterioration in image quality over time and so generally at least one cleaning system is necessary.
On the other hand, various methods of removing spherical toners are proposed. For example, if the photosensitive carrier is cleaned with a blade it is critically important how the frictional force at the blade nip with the photoreceptor surface, there on which the residual toner particles are present after transfer, is controlled, and thus a method of applying lubricant particles to the blade surface is proposed (see e.g., JP-A No. 4-212190). According to the method, the cleaning is indeed improved initially, but the lubricant particles on the blade surface may be exhausted during use for an extended period of time, causing improper cleaning. Alternatively, a method of applying direct current and alternate current bias voltages to the cleaning blade is proposed (see e.g., JP-A No. 5-265360). However, the amount of static charge on the toner remaining after transfer varies according to the amount of static charge on the developer toner, transfer conditions, the environment of use, or the kind of images formed, and hence the application of a voltage does not assure complete cleaning. Also the cleaning bias may sometimes accelerate deterioration of the photoreceptor surface, reducing the life of the photoreceptor.
Alternatively, increasing the pressure between cleaning blade and photoreceptor is proposed (see e.g., JP-A No. 4-001773), but although such an increase in pressure initially improves the cleaning performance significantly, however, if the material and physical properties of the blade are not examined thoroughly, it may cause defects in the blade resulting in incidences of improper cleaning. Also if an organic photoreceptor is used, the amount of the abrasion of the photoreceptor may increase, reducing the life of the photoreceptor.
On the other hand, as an approach for improvement from the developer perspective, adding a fatty acid metal salt to the toner is proposed (see e.g., JP-A No. 2000-89502). However this method, although effective in reducing the frictional force at the nip portion between the cleaning blade and the photoreceptor, may decrease the amount of static charge on toner significantly due to the addition of a fatty acid metal salt, increasing the likelihood of fogging and toner scattering during image development, thereby decreasing image quality.
Further, a method for the addition of a higher alcohol or a higher fatty acid to the toner is also proposed (see e.g., JP-A No. 63-188158), wherein suppression of toner spotting is achieved by use of a higher alcohol having 30 to 300 carbon atoms is discussed. Suppression of comets and filming on the surface of a photoreceptor by addition of a higher alcohol onto blade cleaning systems by various methods is also examined (see e.g., JP-A Nos. 6-282096 and 9-6049). The suppression is explained therein as the result of the fact that, during cleaning, the higher alcohol or fatty acid forms a film of a releasing agent on the surface of the photoreceptor, thereby suppressing filming of the toner and other toner constituent materials directly on the photoreceptor. However, it is necessary to add a large amount of higher alcohol or higher fatty acid in order to form a sufficiently thick lubricant coating, the accompanying effect of which is shown to be improper charging of the toner and a decrease in the surrounding stability (see e.g., JP-A No. 2001-42562). Therefore, a use of a higher alcohol or higher fatty acid having a number of carbon atoms in the range of 21 to 29 is proposed as an improved method of forming a lubricant coating on the photoreceptor easily with an addition of a small amount. However, such a higher alcohol softens readily, and whilst the lubricant surface may be formed more easily, the use of such a higher alcohol often leads to: problems of significant staining of the development sleeve, charged blade, and the like, by the carriers in two-component developers or by the one-component developers; or decrease in the charge retention of the developer.
If the shape of toner is made more spherical to raise the transfer performance when using smaller-diameter toners for the purpose of obtaining higher-quality images, it becomes more difficult to clean the photoreceptor, and thus it is necessary to remove the toner remaining on the photoreceptor after transfer, for example, by raising the linear pressure of the blade. However, raising the linear pressure also causes the problem of accelerated abrasion of the photoreceptor and the blade. Accordingly, it is necessary to improve the cleaning ability without a sacrifice in the electrostatic performance and the ability to retain electrostatic charge.
In particular, in a system for forming color images, wherein an intermediate transfer body is used for transferring images, two kinds of transfers are required, i.e., a primary transfer of transferring an image of the latent image bearing body onto the intermediate transfer body and a secondary transfer of retransferring the image on the intermediate transfer body onto a recording medium. Hence, the cleaning ability with respect to remaining toner on the latent image bearing body as well as on the intermediate transfer body becomes important. In particular, requirements become stricter for color image forming, as it is necessary to remove toners of multiple colors remaining on the intermediate transfer body. Furthermore, tandem system in which latent image bearing bodies and developer bearing bodies corresponding to the four colors of toner are provides and images thereon are transferred either via an intermediate transfer body or directly onto a recording medium, are advantageous from the viewpoints of total transfer efficiency and printing speed. However, such systems should have a corresponding high speed cleaning process compatible with the high-speed processing.
Further, for obtaining high-quality images, where there is an addition of some microparticles of a fatty acid metal salt, higher alcohol, higher fatty acid, or the like for improving the cleaning ability, this is often accompanied with irregularity in closely solid images and half tone images due to inappropriate transfer. These irregularities stand out more in color images with high image density, resulting in a marked decrease in image quality.